In packet-based systems, where the arrival timing of a packet is not known a-priori, there is a need to detect an incoming packet to trigger events in the receiver, such as the synchronization chain. Among the W-LAN (wireless local area network) systems developed in recent years, there are systems that have a wide bandwidth (“BW”) and are based on multicarrier modulation. These systems have a system timing that is very fast in absolute terms, and the same is anticipated with respect to next-generation cellular systems, where sampling rates will be on the order of many tens of Msps. In light of this background, it is necessary to have algorithms for packet detection that are (1) of limited computational complexity to allow fast processing, (2) not prone to false alarms, (3) that detect promptly packets even at the lower edge of the operating SNR (signal-to-noise ratio) region, and (4) if based on a training sequence, then the same sequence has to be BW-efficient and have low PAPR (peak-to-average power ratio).
To assure real-time processing, the packet detection algorithm must be relatively simple, and for this reason it is usually based on the auto-correlation of a given segment of the incoming signal. This guarantees better performance than algorithms that simply monitor the incoming signal energy. In particular, recent implementations seem to favor the use of a short repetitive pattern, used both for packet detection and for coarse frequency offset compensation. This short repetitive pattern is often referred to as utilizing “short training symbols” to indicate that one time period of the sequence is shorter than one OFDM (orthogonal frequency division multiplexing) symbol.
There are, however, drawbacks associated with using a short repetitive pattern, in that the algorithm tends to recognize as an incoming packet every repetitive noise pattern. In particular, the algorithms used with a short repetitive pattern are prone to false alarms when (1) there is a DC component in the input (interpreted as repetitive pattern) as has been the case with IEEE802.11a HW implementations, and (2) there is co-channel interference.
There is, therefore, a need for algorithms that can discriminate more effectively between packets and noise or interference. It should be noted that simple digital filtering can be applied to block a DC (direct current) component, but this can degrade somewhat the incoming signal. It should also be noted that, for next-generation cellular systems, co-channel interference problems in the case of frequency reuse factor 1 are expected to be more relevant than for W-LANs, where in many cases you do not have an adjacent cell directly interfering in your operation area.